Despite intensive research, we simply do not understand why the severity of infections caused by Staphylococcus aureus varies so widely. The recent advances in molecular genetics have provided an unparalleled opportunity to understand how S. aureus genes and genetic changes contribute to the overall severity of illness. Having used my K23 to create the world's largest collection of well-characterized bacterial isolates from prospectively identified patients with S. aureus bacteremia, I now propose to use this resource to evaluate determinants of S. aureus virulence. The long-term objectives of this project are to: 1) identify bacterial genes influencing the severity of infection in isolates from a large cohort of patients with S. aureus bacteremia, and 2) ultimately use these genes to identify novel interventions for the control of S. aureus infections. The overall hypothesis of this investigation is that distinct bacterial virulence determinants influence the severity of S. aureus infection. We specifically hypothesize that virulence determinants associated with clinical outcome of S. aureus infection segregate into clonal groups, identified by Multilocus Sequence Typing (MLST), and can be localized in the genome by comparative genetic hybridization (CGH). In order to test this hypothesis, we propose to: 1) Define the allelic diversity of 1000 S. aureus bloodstream isolates using MLST; 2) Define the genomic diversity of a subset of 200 of these S. aureus bloodstream isolates using CGH; and 3) Correlate MLST and DNA microarray results, MLST and clinical outcome, DNA microarray and clinical outcome, and make fully characterized isolates available to the scientific community. The products of this grant will include an increased understanding of genetic diversity in S. aureus and the role of this genetic diversity in determining the severity of infections caused by S. aureus. The full value of the current proposal also includes the potential future payoff and benefit to the research community as a whole if associations between pathogen genotype and clinical outcome, only possible to identify using such a large and clinically well-characterized collection of isolates, can be defined. This work is critical to furthering the understanding of a crucial medical problem because: 1) S. aureus is an emerging pathogen, and 2) interventions to reduce S. aureus morbidity require a better knowledge of the bacterial determinants of severity of infection. Understanding the bacterial genetic determinants of disease severity in S. aureus infections will advance our understanding of staphylococcal pathogenesis and will enable key advances in protecting the public health from this pathogen.